Changes in plant species richness due to land use and nitrogen deposition across the globe

Aim The effects of land use and atmospheric nitrogen (N) deposition on plant species richness are typically studied in isolation. Here, we quantified the combined effects of these two pressures on terrestrial plant species richness at a 0.25º spatial resolution across the globe. Location Global. Methods We first used local monitoring and experimental data to determine the proportional changes in plant species richness in relation to different land‐use types and N addition levels according to a meta‐analytical approach. We then developed a new multi‐pressure species–area relationship (mp‐SAR) model to combine the site‐level responses resulting from the meta‐analyses with global land use and N deposition maps to calculate changes in plant species richness at a resolution of 0.25º across the globe. Results Both pressures combined resulted in an average plant species richness decline of 26% (± 12% SD) across the grid cells. The combined impact was the largest in Europe, with an average decline of 34% (± 8%), and the smallest in South America (16% ± 11%). Overall, species declines due to land use were considerably larger (19% ± 11%) than declines due to N deposition (6% ± 6%). Main conclusions Our new species–area model allows for systematic large‐scale assessments of the combined effects of multiple pressures on species richness based on easily retrievable input data. Overall, land use is a more important driver of plant species richness decline than N deposition, but N deposition may have considerable additional impact in specific regions of Europe, Asia and North America. These findings imply that conservation efforts should simultaneously tackle both pressures to ensure their effectiveness in preserving plant biodiversity.